The present invention relates to a photosensitive member having an undercoat layer.
Recently, amorphous silicon (referred to as a-Si hereinafter) formed by a plasma chemical vapor deposition (referred to as plasma CVD) has been applied to produce a photosensitive member, especially electrophotosensitive members.
A-Si photosensitive members have several excellent properties. But the relative dielectric constant (.epsilon.) of a-Si is so larger (about 12) that it essentially needs a thickness of at least 25 .mu.m to gain a sufficient surface potential for a photosensitive member. In addition, in the production of an a-Si photosensitive member by plasma CVD a long production time is needed because of the slow deposition rate of an a-Si layer. The long deposition time makes it difficult to obtain a homogeneous a-Si layer, with the result that image defects such as white spot noises are liable to occur at a high percentage. Further, the cost becomes expensive.
Though many attempts to improve the above noted defects have been made, it is not preferable to make the layer thinner.
On the other hand, an a-Si photosensitive member has additional defects such as weak adhesive strength between a-Si layer and electroconductive substrate, and poor resistances to corona, external circumstances and chemicals.
It has been proposed to use an organic polymeric layer which is produced by plasma polymerization (referred to as OPP layer hereinafter) and is arranged as an overcoat layer or an undercoat layer in order to solve the above problems. The former is proposed for instance, in U.S. Pat. No. 3,956,525 and the latter is done in Japanese Patent KOKAI No. 63541/1985.
It is known that an OPP layer can be produced from various kinds of organic compound such as ethylene gas, benzenes, aromatic silanes and the like (e.g. Journal of Applied Polymer Science Vol. 17, 885-892 (1973), by A. T. Bell et al.). However, the OPP layer produced by these conventional methods is restrictively used as an insulator. Therefore, the layer is considered as an insulating layer having an electrical resistance of about 10.sup.16 .OMEGA..cm as an ordinary polyethylene layer or at least similar to such a layer.
Recently, there has been proposed a layer comprising diamond-like carbon in the semiconductor field. But charge transportability thereof has not been suggested at all.
Japanese Patent KOKAI No. 136742/1984 discloses a photosensitive member having a carbon layer of 1-5 .mu.m formed on an a-Si layer, which is applied on an Al substrate in order to prevent Al-diffusion to the a-Si layer at the exposure of light.
Japanese Patent KOKAI No. 61761/1985 discloses an a-Si photosensitive member coated with a diamond-like carbon layer insulator of 500 .ANG.-2 .mu.m thick as a surface protecting layer to improve corona discharge resistance of the photosensitive member and mechanical strength. The carbon layer is so thin that a charge can pass through the layer by the tunnel effect so the layer itself does not need a carrier transporting ability. It does not refer to the carrier transporting ability of the OPP layer. Further, it does not suggest the solution of the essential defect of the a-Si photosensitive member.
U.S. Pat. No. 3,956,525 discloses a photosensitive member consisting of a substrate, a sensitizing layer, an organic photoconductive electrical insulator and a glow discharging polymer layer having a thickness of 0.1-1 .mu.m in the above order. This polymer layer is provided to cover the surface so as to stand up to wet development as an overcoat. Carrier transportability of the layer is not suggested.
Japanese Patent KOKAI No. 63541/1980 discloses a photosensitive member comprising an undercoat layer composed of a diamond-like carbon and having a thickness of 200 .ANG.-2 .mu.m and an a-Si photoconductive layer formed on said undercoat layer. This undercoat layer is formed to improve adhesion of the a-Si layer to the substrate. The undercoat layer may be so thin that a charge moves through it by tunnel effect.
As mentioned above, photosensitive members have been proposed which comprise an undercoat layer or an overcoat layer composed of an electrically insulating OPP layer, a diamond-like layer and the like, but charge transportability is basically attributed to the tunnel effect and the phenomena of dielectric breakdown.
The tunnel effect is caused owing to the passage of electrons, when the thickness of an insulating layer is very thin (generally at an Angstrom order).
The dielectric breakdown phenomenon is where existing small amounts of charge carriers are accelerated by an electric field to gain sufficient energy to be capable of ionizing atoms in the insulator, with the result that carriers increase by the ionization, and the carrier increases geometrically by the repetition of the same process. This phenomena occurs at a high electric field (generally more than 100 v/.mu.m).
In the case of a photosensitive member having laminated layers of an insulating layer and a semiconducting layer, charges generated in the semiconducting layer move through in the layer under an electric field, but they can not pass through the insulating layer under a low electrical field. If the insulating layer is thin, it is ignored as a surface potential or it does not adversely affect properties of photosensitivity because of negligible development influence. Further, even if charges are accumulated on the insulating layer by repeated use to give a higher potential, the potential in the electric field does not increase above a constant level (e.g. 100 V/.mu.m) because of dielectric breakdown.
For example, when an insulating layer comprising insulating materials capable of causing dielectric breakdown at 100 V/.mu.m is formed at a thickness of 0.1 .mu.m, the increase of the residual potential based on the repetition is only 10 V.
According to the above reasons, it is understood that if a conventional insulating layer is used for a photosensite member, the thickness of the layer has to be less than about 5 .mu.m, or else the residual potential based on the insulating layer increases to more than 500 V causing an overlap of the copied image to occur.
As aforementioned, a conventional organic polymer layer in a photosensitive member is used as an undercoat layer or an overcoat layer, which probably requires no carrier transporting ability, and is used from the viewpoint that the layer is insulant. Therefore, only an extremely thin layer, at most 5 .mu.m, is proposed. The carriers generated in the photosensitive layer pass through the organic polymer layer by a tunnel effect. In the case that the tunnel effect cannot be expected the layer is only used so as to be so thin that the residual potential is negligible.
It has been found that the organic polymer layer, which has been considered inherently insulant, has a carrier transporting capacity at some range of hydrogen content.
Further it has been found that when an undercoat layer consisting of a polarity controlled hydrogen-containing carbon layer is arranged on an electroconductive substrate, adhesivity to the substrate is improved and injection of charge from the substrate is prevented.